In the Regge limit, the squared centre-of-mass energy \(s\)  is much larger than the momentum transfer \(t\) , \(s\gg |t|\) , and
\(2\rightarrow 2\)  scattering amplitudes are dominated by gluon exchange in the \(t\)  channel. Contributions which do not feature gluon exchange in the \(t\)  channel
are power suppressed in \(t/s\) . At tree level we can write the \(2\rightarrow 2\)  amplitudes in a factorised way. For example,
the tree amplitude for gluon-gluon scattering \(g_1\, g_2\rightarrow g_3\,g_4\)  may be written as [1]}, [2]},
\(\mathcal {M}^{(0)}_{4g} =\left[g_s (F^{a_3})_{a_2c}\, C^{g(0)}(p_2, p_3) \right]{s\over t} \left[g_s (F^{a_4})_{a_1c}\, C^{g(0)}(p_1, p_4) \right]\, ,\) 
